Low pass filters are utilized in radio frequency (RF) front end circuits to remove undesired harmonics or spurious signals resulting from the mixing of signals in the RF front end circuit. It is important to design low pass filters with a wide stopband and a high stopband rejection to improve the linearity of the RF front end circuit and to reduce the bit-error-rate (BER) in high date rate communication systems. It is also important for the low pass filter to have a compact size and an integration ability to reduce cost of the RF integrated circuit.
While stepped impedance low pass filters and open stub low pass filters are commonly used for low pass filter implementation, the stepped impedance low pass filters and open stub low pass filters have a disadvantage in that both implementations provide a gradual cut-off response. By increasing the number of sections of the step impedance low pass filters or the open stub low pass filters, the rejection characteristics of the low pass filters can be improved. However, increasing the number of sections will increase passband insertion loss as well as the physical dimensions of the low pass filters. Some low pass filters employ a semi-lumped element composed of a lumped capacitor and a section of transmission line to cope with this problem. However, the use of multiple lumped elements will increase the component cost of the low pass filters as well as increasing the assembly cost of the RF integrated circuit, especially for low pass filters operating in a high RF frequency range. Multi-section low pass filters using microstrip line and microstrip elements such as interdigital capacitors, coupled lines and stepped-impedance hairpin resonators are other effective approaches. A stepped-impedance hairpin resonator allows design of low pass filters of relative smaller size and with an additional zero point in the stopband (e.g., achieved through an additional electric coupling path). In addition, defected ground structure (DGS) microstrip line structures have been utilized in low pass filters to implement wide stopband. However, the DGS microstrip line structure introduces disadvantages. For example, DGS microstrip line structure increase radiation due to a partially open ground plane, requiring a metallic enclosure to shield the DGS structure, and thereby increasing the cost of the low pass filters.
Generally, most of the prior art low pass filter designs are quite bulky, which is not economic to use with other circuits, especially they are not possible to be integrated with RF integrated circuits. Active low pass filter designs have been proposed to overcome the cost and integration issues. However, active low pass filters usually contribute more noise than their passive counterparts and have very limited stopband performance. Another important drawback is that active low pass filters consume power.
Thus, what is needed is a low pass filter design which successfully and simultaneously addresses the problems of compactness, in-band matching, insertion loss, stopband rejection and stopband bandwidth. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.